Muffled conversations initially had researchers at a San Diego aquarium baffled, but when a diver thought he was told to get "out" of the water they realised the chatter was coming from a beluga whale.
Noc was about a year old when he was captured off the Pacific coast of Canada in 1977. He was kept in an open-ocean pen at the US National Marine Mammal Foundation in San Diego, California.
Beluga whale 'speaks' to humans
When staff at a marine foundation in San Diego first heard the garbled sounds underwater they assumed two divers were having a conversation. Turns out it was a whale called NOC.
After seven years at the foundation he began, spontaneously, to make unusual sounds, a report in the latest issue of the journal Current Biology reported.
"We interpreted the whale's vocalisations as an attempt to mimic humans.
"Whale vocalisations often sounded as if two people were conversing in the distance just out of range for our understanding," the authors said.
"These 'conversations' were heard several times before the whale was identified as the source."
The researchers realised it was Noc when a diver surfaced and asked: "Who told me to get out?"
It was realised the word "out", which was repeated several times, had come from Noc.
After that, his speech-like sounds were recorded in air and underwater. Noc started making the sounds in 1984 after seven years in captivity, and continued making them for another four years.
He died five years ago but researchers have now analysed the archived sound recordings.
It was the first time acoustic recordings showed how such sounds emulated speech and deviated from the usual calls of the species, the researchers said.
They show the speech-like sounds were at fundamental frequencies several octaves lower than normal whale sounds, and much closer to those of the human voice.
Also, whales talk to each other by blowing air through their noses rather than using a larynx, as people do.
"We do not claim that our whale was a good mimic compared to such well-known mimics as parrots or mynah birds," the researchers said.
"However, the sonic behaviour we observed is an example of vocal learning by the white (beluga) whale.
"It seems likely that Noc's close association with humans played a role in how often he employed his human voice, as well as in its quality."